U.S. Pat. No. 4,193,945, assigned to Montedison, S.p.A., describes self-extinguishing polymeric compositions containing, as a flame-retardant additive, a mixture of an ammonium phosphate and of a nitrogen-containing compound selected from a group of compounds comprising also a product of condensation between N,N'-ethyleneurea and formaldehyde.
Such compositions are endowed, on one hand, with excellent flame-retardant properties, but exhibit, on the other hand, a low resistance to heat and to thermo-oxidation. In fact, in the molding of manufactured articles at temperatures higher than 200.degree. C., those polymeric compositions undergo color changes with gradual turning from white to light hazel, dark hazel and finally brown. Furthermore, the specimens obtained from such compositions and maintained for a certain time-period at high temperatures in a forced draught furnace lose their uninflammability after a certain number of days (the oxygen index decreases from 30 to less than 25) and become brittle even if they had been prepared from antioxidant-containing polymeric compositions.